This invention relates generally to solid state light emitters and, more particularly, relates to semiconductor light emitter devices including, but not limited to, light emitting diodes (LEDs), laser diodes, vertical cavity surface emitting laser devices (VCSELDs), resonant cavity (RC) LEDs and similar devices.
Modern data communication systems and networks are increasingly being implemented with optical technology, more specifically with optoelectronic technology. One important element of any optically-based, high bandwidth communication system is the light emitting device. In order to achieve high modulation frequencies of the emitted light it is important that minority carrier recombination occur in an efficient and rapid manner. Put another way, it is desirable to achieve a high modulation frequency of the output light with high radiative efficiency. Prior art light emitting devices have been limited in fulfilling this need.
Current limitations of LEDs include the limited modulation frequency ( less than 1 GHz), limited output power ( less than 1 mW), low optical fiber coupling efficiency ( less than 20%), and broad spectral line width. These properties limit the use of LEDs to applications such as short haul  less than 650 Mbs optical links, as well as to indicator lamps and similar illumination applications.
Current limitations of laser diode devices include low manufacturing yield and hence high cost, non-ideal modulation characteristics (chirping), and a general lack of enablers to manufacture surface emitting lasers that emit in the 1.3 to 1.5 micron wavelength range. These properties limit the current large scale application of laser diodes to discrete cw laser/electro-optically modulated optical links operating at 10 Gb/sec using wavelength division multiplexing (WDM), such as four channels each operating at 2.5 Gb/sec. Thus, since lasers are bandwidth-limited, high bit rate communications rely on parallelism. However, the use of parallelism increases both the cost and complexity of the communication system.
Researchers have recently increased the bandwidth of LEDs, using specialized processing techniques, to 1.7 GHz, and data rates as high as 1.7 Gb/sec have been demonstrated (with a bit error rate  less than 10xe2x88x929 using xe2x88x9223 dBm input power). However, the external optical efficiency was limited to 2.5 xcexcWatts/mA. Also, in general, optical efficiency decreases with higher speed LED design and fabrication techniques.
A consistent problem that has faced researchers is related to the conflicting goals of attempting to provide device material that exhibits high radiative recombination efficiency, in order to maximize the signal to noise ratio and output power, while at the same time attempting to provide the same device material (with high radiative recombination efficiency) and a short minority carrier lifetime, in order to maximize the frequency at which the generated light can be modulated with the desired information. As can be appreciated, and has been observed in the literature, a point is reached at which one of the radiative efficiency or the modulation frequency will begin to decline at the expense of the other.
It is expected that in the near future system designers will require emitters that can be modulated in excess of 10 GHz, that exhibit high output power at 10 GHz (about 1 mW), and that exhibit a narrow spectral line width and low dispersion losses.
Thus, there is a well recognized need to develop faster and higher power optical emitters.
It is a first object and advantage of this invention to provide an improved light emitter that fulfils the foregoing need.
It is a second object and advantage of this invention to provide a light emitting device that includes a light emitting region, such as a LED, laser diode, or VCSELD; a minority carrier barrier, such as a resonant tunneling structure or other potential barrier (such as a triangular, square or parabolic potential barrier structure formed by compositional grading or impurity concentration grading); and a region that exhibits a low radiative recombination efficiency as well as a short minority carrier lifetime, such as a region comprised of a low temperature grown material or a Schottky barrier.
It is a further object and advantage of this invention to provide a light emitting structure wherein light emitting efficiency considerations may be decoupled from modulation frequency considerations, wherein one region of the structure may be optimized to increase minority carrier radiative recombination efficiency and lifetime, while another region of the structure may be optimized to decrease non-radiative minority carrier lifetime.
The foregoing and other problems are overcome and the foregoing objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
This invention fulfils the foregoing and other needs by providing a light emitter, such as a LED, that exhibits (a) a bandwidth in excess of 10 GHz without degraded output power; (b) that operates by surface emission, thereby enabling fabrication in an array format; (c) that provides an ability to be modulated either electrically or optically (with optical gains greater than one); and (d) that has an ability to be fabricated in an all-optical embodiment, wherein the device operates entirely by optical input without requiring electrical connections.
A light emitting device is constructed so as to provide a first part that includes a source of excess minority carriers (over and above equilibrium) and/or excess electron-hole pairs; a second part, coupled to the first part, that includes a minority carrier barrier; and a third part, coupled to the second part, that includes a region that exhibits a low radiative recombination efficiency and short minority carrier lifetimes. In response to a first stimulus minority carriers are constrained by the second part to remain in the first part, leading to an increase of minority carrier radiative recombination in the first part and an increase in light emission; while in response to a second stimulus the minority carriers are enabled to cross the minority carrier barrier of the second part to enter the third part, leading to a decrease of minority carrier radiative recombination in the first part and a decrease in light emission. In certain embodiments the first stimulus includes an absence of an electrical signal applied between the second part and the third part, and the second stimulus comprises a presence of the electrical signal applied between the second part and the third part. In other embodiments the first stimulus induces a change in the electric field in the second part that is generated by optically induced electron-hole pairs in the second part, and the second stimulus includes the electric field that was; present prior to the first stimulus. In another embodiment the first stimulus includes an absence of a change in the electric field in the second part, and the second stimulus includes a presence of the change in the electric field generated by the optically induced electron-hole pairs in the second part. In certain embodiments the first stimulus (or the second stimulus) can be the presence of modulating light incident on the second part and a resultant decrease in band bending.
The first part can include, by example, a light emitting diode, a laser diode, a vertical cavity surface emitting laser device, or a resonant cavity light emitting device. In another, all optical embodiment the first part includes a material that, as a result of optical pumping, provides a photoluminescent emission or a laser-type or laser-like emission. The second part can include a resonant tunneling structure, such as quantum well, or one of a triangular, square or parabolic potential barrier structure formed by compositional grading or impurity concentration grading. The third part can include a normal temperature grown or a low temperature (LT) grown material, such as a doped LT GaAs layer, and/or an undoped layer with a Schottky barrier contact.
The light emission can be produced as a result of an electrical bias applied to the light emitting device, or as a result of the above-mentioned optical pumping that results in the photoluminescent emission or the laser-type or laser-like emission.
It is shown that embodiments of this invention are capable of exhibiting optical gain (e.g., gain in excess of 1), and an optical semiconductor light emitting device with optical gain (SLEDOG) is thus made possible by these teachings, as is an all-optical device that does not require electrical inputs.
A method is also disclosed for fabricating a light emitting device. The method includes steps of (a) providing a semiconductor light emitting structure that contains a source of excess minority carriers; (b) forming a first structure over a surface of the semiconductor light emitting structure, where the first structure is formed from a material that functions as a minority carrier barrier upon an application of a predetermined stimulus, thereby constraining minority carriers to remain in the semiconductor light emitting structure and resulting in minority carrier radiative recombination therein; and (c) forming a second structure over the first structure, the second structure containing a material that exhibits a low radiative recombination efficiency and a short minority carrier lifetime. The second structure removes minority carriers that cross the first structure, after escaping from the semiconductor light emitting structure, in the absence of the predetermined stimulus.